Method of spectrally camouflaging a structure with a surrounding environment

ABSTRACT

Disclosed are methods to produce composite materials, which contain customized mixes of nano- and/or micro-particles with tailored electromagnetic spectral properties. In some defense-related applications, the use of such materials enables an improved spectral match between different structures, such as vehicles or buildings with the surrounding environment at least in the VIS and NIR wavelength range. This can camouflage the structures, and reduce the detectability thereof by ground-, air- or space-based multi-spectral long-range imaging systems, including aircrafts, drones, and satellites, and thus, generally delay, complicate, or eliminate detection or classification of the camouflaged structures.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application are hereby incorporated by reference under 37 CFR 1.57. This application is a CON of Ser. No. 16/387,206 (FILED Apr. 17, 2019), which application is a CON of Ser. No. 14/831,813 (filed Aug. 20, 2015, now U.S. Pat. No. 10,280,626), which application claims priority to U.S. 62/041,303 (filed Aug. 25, 2014).

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to methods of creating composite materials with tailored spectral properties in the visible and invisible electromagnetic (EM) wave spectrum using a plurality of discrete populations of nano- and/or micro particles and/or nano- and/or micro cavities.

In some embodiments the invention relates to structural elements, which enable the design and construction of buildings or other confined spaces with improved thermal and energetic properties. In particular, the invention enables the construction of buildings, which absorb less solar radiation per given surface area. This results in lower temperatures inside buildings or other confined spaces, increased time to reach higher temperatures inside such buildings or predominantly confined spaces, and/or in reduced power requirements for air-conditioning systems.

Description of the Related Art

Many technical applications require to control the temperature inside predominantly enclosed spaces or, as a related aspect, the control the electromagnetic spectral properties of materials and surfaces. In some instances it may be desirable to keep the temperature inside predominantly enclosed spaces low, or even as low as possible, whereas in other applications the opposite may be the case. In yet other instances, for example related to achieving broadband camouflage effects, the electromagnetic spectral “appearance” may be of primary interest.

The broadband electromagnetic spectral properties of surface materials (within a certain wavelength range) can affect:

-   -   (a) how a material/surface visually appears,     -   (b) how readily visible and invisible electromagnetic radiation         is absorbed from a surface and thus gets converted into thermal         energy, and     -   (c) how rapidly a surface can lose thermal energy by emitting         electromagnetic waves at thermal wavelength.

The invention relates in particular to the design and manufacturing of composite materials, which have tailored and improved Ultraviolet (UV), visible (VIS), Near Infrared (NIR), Mid Infrared (MIR), and Far Infrared (FIR) properties, by containing tailored mixtures of nano- and micro particles. Henceforth the term ‘light’ shall be understood to apply for all of these spectral ranges, not only the portion visible to humans.

Embodiments herein may be described within the context of a range of electromagnetic frequencies ranging from UV to FIR, since these are of increased importance for visual appearance, thermal management, and camouflage. However, the invention should not be limited to these frequencies, as similar arguments can be made for other electromagnetic frequencies.

Thermal management of predominantly enclosed spaces, including buildings, can be a problem in geographic locations with relatively high levels of solar radiation, specifically in conjunction with high energy cost, which makes in some countries the use of permanent air-conditioning financially problematic for a significant segment of the population.

A considerable portion of the energy, which contributes to indoor temperatures, is the result of absorbed solar radiation by the building envelope. This effect can be true in tropical and subtropical regions, but it also applies in moderate climate zones, for example, during summer or times of increased relative temperature (e.g., in North America, Central and Southern Europe).

Related problems apply also to predominantly open spaces, such as large cities and densely populated areas in tropical and subtropical regions, where concrete and asphalt surfaces can significantly contribute to the common “heat island” effect in such areas.

Thus, reducing the amount of solar radiation, which is absorbed by the surface elements of predominantly enclosed spaces, including buildings, can contribute to reducing indoor temperature and/or reduce the required power for air-conditioning. Furthermore, widespread application of such advanced passive cooling methods can contribute to mitigate the heat island phenomena observed in large cities.

In some embodiments the disclosed invention can serve to fabricate materials and surfaces, which enable to control to a very high degree independently the spectral behavior in the VIS range and NIR range, i.e., optical color and NIR “color”.

In other embodiments of the disclosed invention, such as inside solar-thermal collectors, the exact opposite is the case: Here the target is to maximize the wavelength range and efficiency of conversions of solar radiation into thermal energy.

In yet other embodiments the disclosed invention, a goal is to achieve a close spectral match of surfaces against a given background, i.e. to “blend in” with the environment, preferable over a wide spectral range. Such effects are desirable in defense-related applications, for example surfaces of buildings (bunkers) or combat vehicles, to avoid, delay, or complicate optical, NIR, IR, or FIR (i.e., thermal) detection, including by ground-, air-, or space-based long-range multi-spectral imaging systems.

SUMMARY OF THE INVENTION

For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described herein. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.

In an embodiment, a method of creating a composite material is provided. The composite material includes a plurality of populations of nano- or micro-particles and/or of nano- or micro-cavities of predominantly distinct size distributions, shape distributions, chemical compositions, crystal structures, and crystallinity distributions. The plurality of populations are predominantly embedded in a carrier material, in such composition ratios, with such typical properties per population, and in such density relative to the carrier material, that a specific targeted broadband spectral reflectance at least within the VIS and NIR range is approximated, which is at least partially different from the spectral properties of each individual population within the plurality of populations and the carrier.

All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

A brief description of the drawings shall serve to summarize some principles of the disclosed invention.

FIG. 1 illustrates the average solar spectrum, which arrives at the average distance between the sun and the earth (i.e., at the outer edge of the atmosphere) as well as maximum average normal irradiance at sea level. More than 50% of the power of the solar radiation is contained within the NIR region.

FIG. 2 illustrates schematically the spectral reflectance of a typical material used to coat surfaces in order to achieve a certain color and/or sealing against the water. While clearly a certain visual appearance is achieved, the material exhibits increased spectral absorption (decreased reflectance) in the NIR range resulting in a relatively large amount of solar radiation, which is converted to thermal energy.

FIG. 3 illustrates schematically the spectral reflectance of a typical material with somewhat improved NIR properties resulting from relatively rudimentary approaches to address this problem. Sometimes these materials are partially marketed as “cool products” (tiles, paints etc.). However, such other approaches typically utilize embedded particles with random or at best weakly controlled and broad size distribution. The result is that still a substantial portion of the solar radiation gets converted to thermal energy.

FIG. 4 illustrates simplified and schematically the disclosed method of creating a composite material, which contains a plurality of populations of nano- or micro-particles and/or of nano- or micro-cavities of predominantly distinct size distributions, shape distributions, chemical compositions, crystal structures, and crystallinity distributions, which are predominantly embedded in a carrier material, in such composition ratios, with such typical properties per population, and in such density relative to the carrier material, that a targeted broadband spectral reflectance at least within the VIS and NIR range is achieved.

In particular, the composition of said plurality of populations of nano- or micro-particles and/or of nano- or micro-cavities is determined by performing an optimization loop in a high-dimensional space comprising at least taking portions of relative amounts c_(i) [0, 1] from a plurality of reservoirs R_(i) corresponding to a plurality of said populations of nano- or micro-particles, mixing said portions, and embedding said mixture of said nano- or micro-particles in a carrier material thereby creating a composite. An amount of said composite material is made with sufficiently large surface to determining its broadband spectral properties at least within VIS and NIR. Subsequently an optimization algorithm determines a composition vector _(j)c of a mixture of said nano- or micro-particles taken from each reservoir R_(i) during a next iterative step j. The process is repeated until a criterion, which is a measure for the degree with which the spectrum obtained from the composite produced during the last iteration does match the target spectrum, has been reached or surpassed. Subsequently larger quantities of said composite are produced for various applications.

FIG. 5 illustrates schematically the spectral reflectance of a typical composite material, which can be produced with the disclosed method. In this particular example the composite material has a specific visual (non-white) color and a very high NIR reflectance. The result is that most of the solar radiation in the NIR spectrum does not get converted to thermal energy.

FIG. 6 illustrates schematically the spectral reflectance of a typical composite material, which can be produced with the disclosed method. In this particular example the composite material has a specific visual (non-white) color. The NIR reflectance is very high in certain regions of the NIR range but exhibits imperfections, i.e. drops at certain narrow wavelength rages. This may be caused by the specific available selection and the types of populations of nano- or micro-particles, which does not allow to produce completely even broadband NIR high reflectance. However, the disclosed method was in this particular example still able to generate a useful material with increased, or nearly perfect properties since the imperfections (reflectance drops) coincide with the spectral gaps in the natural solar NIR spectrum typically arriving on the surface of earth. Thus, still most of the solar radiation in the NIR spectrum does not get converted to thermal energy.

DETAILED DESCRIPTION

Solar radiation, which arrives at the surface of the earth, has a peculiar spectral distribution. This is the result of the spectral distribution of the radiation emitted by the sun, which arrives at the outer edge of the atmosphere (in first order Black-body radiation following Planck's law) and the spectrally selective distribution of scattering and absorption within the earth' atmosphere.

The electromagnetic spectrum can conceptually be divided in Ultraviolet (UV), visible (VIS), Near Infrared (NIR) [0.7 μm-3.0 μm], Mid Infrared (MIR) [3 μm-50 μm], and Far Infrared (MIR) [50 μm to 1 mm]. The exact definition of the boundaries of NIR, MIR, and FIR are somewhat arbitrary and different values are used by different scientific communities. (For example, sometimes the NIR range is defined smaller and an additional Short Wavelength IR SWIR is defined.) For physical reasons (the corresponding black body temperatures) we will henceforth follow the values given above.

As illustrated in FIG. 1, most of the solar irradiance (radiation power density) is contained in the wavelength range between 0.25 ™ and 3.0 μm. This Figure is based on the reference solar spectral irradiance ASTM G-173. (Additional information regarding spectral properties of solar energy can be found, for example, at the National Renewable Energy Laboratory (NREL) website and www.nrel.gov.) Integrated over the entire spectral range, the irradiance (power density of the radiation), which arrives from the sun at the average distance between the sun and the earth (i.e., at the outer edge of the atmosphere) is approximately 1.36 kW/m². Determined by the chemical composition and density of the air and furthermore influenced by humidity (incl. clouds), dust, pollution levels, latitude, time of day, date, etc., the solar radiation is attenuated while passing through the atmosphere and the spectrum is filtered (some bands are suppressed). This is also illustrated in FIG. 1. Under best-case conditions, the peak levels of the irradiance at sea level is about 900 W/m².

As a reference, in the southwest of the United States, the average energy density per day on a horizontal plate is about 5 to 6 kWh/m²/day on a horizontal plate and >9 kWh/m²/day on a surface, which is 2-axis tracking to remain normal to the incidence (Source: National Solar Radiation Data Base).

The part of the spectrum, which is visible (VIS) to humans, is approximately within the wavelength range between 380 nm and 700 nm. While this visible range corresponds to portion of the spectrum with the highest irradiance, it contains only approximately 45% of the solar power, which arrives at the surface of the earth. The Ultraviolet (UV) portion contains only about <4%, but the range between 750 nm and 2.5 m, which is within the NIR range, contains about 51% of the solar irradiance. (The ratios are good approximates. More precise values depend on numerous other parameters.)

In order to minimize the temperature increase of an object, which is exposed to (sun)light, ideally all of the incident power should be reflected by its surfaces. Clearly, the best performance, i.e., the least relative amount of absorbed power (i.e., the ratio of absorbed power to arriving radiation power), would be expected from a material or a coating, which acts as a broadband reflector, i.e. is highly reflective in UV, visible, and IR (incl. NIR, MIR, FIR), essentially a broadband mirror or at least a diffusely reflecting broadband “white”. Achieving such superior broadband reflectivity constitutes one embodiment of the disclosed invention. Such broadband white or “super-cool” surfaces may be particularly desirable in tropical and or subtropical regions but also e.g. in the southwest of the United States.

In some instance, however, building codes may impose constraints on usable (visual) color of buildings, or certain colors are preferred for esthetic reasons.

Thus, in another embodiment of the disclosed invention, materials are created whereby one is able to control, predominantly independently, the reflectivity of a surface in both the VIS and NIR parts of the spectrum, thus one can at least partially independently control the optical appearance and the IR “appearance”.

Most conventional materials, such as those used for buildings, as well as regular paints, have considerable NIR absorption as illustrated in FIG. 2. However, as pointed out above, at least half of the solar power is contained in the NIR spectrum. Thus, optimizing the reflectivity in the NIR region has the potential to reduce the absorbed solar power to at least 50%, even for surfaces, which (as the most extreme) visually appear to be black.

Relatively rudimentary approaches to address this problem have been reported and are partially marketed as “cool products” (tiles, paints etc.). However, such earlier approaches typically utilize embedded particles with a random size distribution or at best a weakly controlled and broad distribution.

Usually powders of various metal oxides (e.g. TiO₂, YMnO₃, Al₂O₃) or semiconductor oxides such as SiO₂ are used, but the size distributions of the particles is not or not well known, or varies depending on the fabrication batch, or the location from which they were collected, and is thus in general not well suited for the indented purpose. The consequence is, that some moderate improvements in the NIR reflectance can be achieved, but the typical results are far from optimal, reaching reflectance values of only around 0.4, but often even considerably less.

Thus, the disclosed invention enables substantial additional improvements in achieving desired spectral properties of material, e.g. high NIR reflectance, if the size distribution of the embedded nano- and/or microparticles is intentionally tailored.

In the most general sense the “color” of a surface, i.e., its broadband spectral reflection and absorption, is the result of its chemical composition and, if present, of the size of contained particles and/or cavities, if they are within a certain size ratio compared to the wavelength.

More precisely, the spectral properties depend on the electronic structure (energy levels and energy bands of electrons, including any band gaps) as well the microscopic (on the scale of the wavelength) distribution of the refractive index n. The refractive index can be derived from the material's macroscopic relative permittivity ε_(r), and its relative permeability μ_(r) according to n=(ε_(r)·μ_(r))^(0.5). At optical frequencies most naturally occurring materials have a μ_(r) very close to 1, thus the refractive index is proximately n≈ε_(r) ^(0.5). Relative permittivity is a measure for electric polarizability, again as a result of specific electronic configurations. Thus, ultimately, n can also be reduced to variations in the distribution of the electron density and polarizability. (Which is why embedded cavities can have effect on the spectral properties.)

Naturally occurring or synthesized chemicals, i.e. molecules, which are used as dies, have such electronic structures (energy levels), which cause highly specific selective absorption of optical wavelength, and thus produce the distinct optical appearance (color).

However, the precise theoretical and/or numerical prediction of spectral properties (at any wavelength range) of arbitrary materials {i.e. any number of chemicals, in arbitrary abundance ratios, and with arbitrary particle size (distribution) and shape (distribution), level of crystallinity, potentially embedded in any other combination of materials} is extremely difficult.

As the most basic case, already more than 100 years ago special solution to Maxwell's equations obtained by Gustav Mie enabled to estimate the (elastic) electromagnetic scattering behavior of small gold particles (as reasonable approximation of so-called perfectly conducting PEC spheres) in solution (water), under the assumptions that (a) the particles are spherical and (b) sufficiently sparsely distributed to consider their scattering behavior independent of each other. (Gustav Mie, Contributions to the optics of diffuse media, specifically colloidal metallic suspensions, Annalen der Physik, 4^(th) series, Vol. 25 (1908) No. 3, pp. 377-445.) Likewise, Lorenz' theory provided an approximation of the electromagnetic wave scattering behavior of sparsely distributed dielectric spheres.

However, both assumptions are not valid for composite materials, which are of practical use as building materials, i.e. have potentially impact on the thermal management of contained spaces. Thus, more advanced models also need consider non-spherical particles or cavities (which are practically the norm), and require the computation of the wavelength-dependent anisotropic scattering behavior of such objects. Moreover, most practical solid materials, or suspensions of particles, which are cast or applied as a liquid or gel and then dried and form a solid, must have high particle densities, which in turn necessitates to consider cascades of multiple scattering among such particles as well as interference effects depending on the material thickness. Realistically, such calculations need to be performed as numerical simulations for sufficient accuracy, but this creates very high computational demands (e.g. at least in part based on Finite-Difference Time Domain solvers for Maxwell's equations).

While such theoretical and numerical methods can provide very helpful guidance for the development of materials with certain spectral qualities, in some cases experimental methods remain to be essential. One such insight is that the overall spectral properties of the material can be far better controlled, if the material consists of a tailored combination of discrete constituents, which contain nano- and/or micro particles of highly distinct size, shape, and electronic configurations.

In few cases will one be able to find natural occurring material, which will have spectral properties that are ideal for a considered application. Thus, by using a tailored mix of nano- and/or micro particles, far better matching of the desired spectral properties of the final material to a target spectrum can be achieved.

Yet again, a forward determination of the optimal mix of nano- and/or micro particles to match a given target spectrum is in general difficult if not impossible to obtain. Thus, as illustrated highly simplified and schematically in FIG. 4, the disclosed invention comprises at least the following steps:

-   -   a) Preparing a plurality k of reservoirs R_(i) with i=[1, k],         each of which contains nano- or micro-particles of distinct size         distribution, shape distribution, chemical composition, crystal         structure, and crystallinity distribution. Preferably, these         distribution functions are predominantly comprised of a single         narrow peak, although multiple peaks may also be acceptable. Of         increased importance is consistency.         -   This desired simplicity of the distribution functions (per             reservoir) does not necessarily imply that their spectral             properties are simple, but it ensures their consistency,             i.e. in a sense the purity of the distributions, which is             what enables reliable and repeatable superposition of their             spectral properties to produce the targeted spectral             properties of the final composite material. This is             important since due to the high density of the various nano-             or micro-particles the electromagnetic scattering behavior             depends on the presence of other particles in the             neighborhood of a given particle (i.e., it is not a             completely orthogonal superposition).         -   In some embodiments the nano- or micro-particles for a             specific reservoir are directly found in nature but             typically have to undergo a rigorous quality assessment             (i.e., experimental determination of said distribution             functions, chemical composition, and crystallinity) to             assure that said distribution functions are maintained.         -   In some embodiments said nano- or micro-particles for a             specific reservoir are selectively filtered from a source of             less refined materials, which typically have varying and             broad distributions. Again, quality assessment is performed             in preferred embodiments in order to assure that said             distribution functions are maintained.         -   In some embodiments said nano- or micro-particles for a             specific reservoir are created by one or more chemical             processes, which tend to predominantly produce nano- or             micro-particles with distinct size distribution, shape             distribution, chemical composition, and crystallinity             distribution.         -   In some embodiments said nano- or micro-particles for a             specific reservoir are obtained by selectively filtering             particles created by one or more chemical processes, which             tend to predominantly produce particles with a wide and or             varying size distribution, shape distribution, chemical             composition, and crystallinity distribution.         -   In some embodiments said nano- or micro-particles for said             various reservoirs come from a combination of said sources.         -   In some embodiments said nano- or micro-particles from some             of said reservoirs will have similar, or even identical             chemical composition and shape distribution functions, but             may vary in terms of size distribution functions among said             subset of reservoirs. For example, there may be reservoirs             with particles with typical average sizes of 25 nm, 50 nm,             100 nm, 200 nm and so on.         -   In some embodiments said nano- or micro-particles from some             of said reservoirs will have similar, or even identical             chemical composition and size distribution functions, but             may vary in terms of shape distribution functions among said             subset of reservoirs.         -   In some embodiments said nano- or micro-particles from some             of said reservoirs will have similar, or even identical             shape distribution functions and size distribution             functions, but may vary in terms of chemical composition or             crystallinity among said subset of reservoirs.         -   In some embodiments the nano- or micro-particles for a             specific reservoir typically have to undergo a rigorous             quality assessment (i.e., an experimental determination of             said distribution functions, chemical composition, and             crystallinity) to assure that said distribution functions             are maintained.         -   Corresponding extensions of this principle should now be             apparent to those skilled in the art. The goal is to             establish preferably wide-spread points in the k-dimensional             parameter space, which is conceptually formed by the             distinct particle properties of the individual reservoirs.             By way of analogy, the reservoirs represent a broadband             “color” palette, although, as mentioned, in general a simple             linear additive “color” mixing, i.e. spectral mixing, is not             possible.     -   b) Creating a mixture of said nano- or micro-particle based on         portions c_(i) taken from each reservoir R_(i). Thus, c_(i)         shall denote the relative contribution [0, 1] of particles from         each reservoir to the final mixture. In many embodiments this         may be a relative contribution by mass or by volume. The entire         ensemble of all c_(i) shall subsequently be referred to as the         composition vector c.         -   In some embodiments of the disclosed invention the process             of taking the correct amount from each reservoir and mixing             these distinct populations of said nano- or micro-particle             distributions is performed by suitable, electronically             controlled equipment.     -   c) Mixing, and/or sintering, and/or dispensing and embedding         said mixture of said nano- or micro-particle in a chosen carrier         material thereby forming what shall be referred to as composite         or composite material. In many embodiments the carrier material         may initially be liquid or gel-like but will subsequently         solidify, for example by evaporation or by an induced chemical         process.     -   d) Forming a solid from said composite, or         -   creating an inhomogeneous solid, a portion of which near its             surface predominantly consists of said composite,         -   or applying a sufficiently thick amount of said composite to             the surface of another typically predominantly solid             material, and         -   the resulting area where the composite is exposed being             large enough to conduct the measurements described in the             next step.         -   In some embodiments the predominantly dry particles from the             different reservoirs (under step c) only mixed and this             mixture is then applied to an at least partially wet bulk             material, which subsequently dries at ambient temperature             (although this process may create some heat), or is baked,             and which thus permanently incorporates said particles at             least into its surface region. In some such embodiments to             this bulk material may be concrete, clay, porcelain, or             other ceramics. In some embodiments some or all of the nano-             or micro-particles in the individual reservoir are in a             suspension prior to mixing.     -   e) Determining the broadband spectral properties of the         resulting surface. In some embodiments this will comprise         -   Exposing the surface to light of predominantly singular             wavelength, e.g. by using a light source of predominantly             singular wavelength (e.g. a tunable laser, VCEL(s), LED(s)),             and/or by using an at least partially broadband light source             (including but not limited to arc lamps, incandescent lamps,             gas discharge lamps, globars or similar Planck radiators,             plasmas, natural solar radiation) and a variable filter,         -   Measurement of the reflectance and/or absorption of the             surface,         -   Scanning the wavelength across the wavelength range of             interest         -   (at least comprising VIS and NIR), and thus         -   producing the entire reflectance and/or absorption spectrum.         -   In some embodiments this will comprise         -   Exposing the surface to broadband EM wave spectrum with             spectral composition (at least comprising VIS and NIR) from             sources including but not limited to arc lamps, incandescent             lamps, gas discharge lamps, globars or similar Planck             radiators, plasmas, natural solar radiation,         -   Measurement of the reflectance and/or absorption of the             surface with a sensor which can discriminate wavelength or             photon energy, and         -   thus producing the entire reflectance and/or absorption             spectrum. In both cases additional steps, such as those             concerning calibration, comparison with reflected spectra             obtained from reference surfaces etc., may be implemented             during this step to ensure sufficient accuracy of the             obtained spectral data.         -   In some embodiments the radiation, which is reflected from             the produced surface, is measured using a sensor which is             positioned within a hollow, highly reflective hemisphere             (with certain known spectral characteristics) or a similar             configuration, in order to capture both direct and diffuse             reflected radiation.     -   f) Determining the deviation of the thereby obtained reflection         or absorption spectrum from a desired target spectrum. In some         embodiments this may only be a derived single scalar, such as a         root mean square error (RMSE), i.e. the square root of the         average squared deltas at each spectral point, which is then         provided to the subsequent optimization algorithm.         -   In some embodiments that quantity may be derived by also             using weighting factor for certain spectral regions in order             to enhance or suppress the influence of said regions on the             subsequent optimization process. In some embodiment the             entire “error spectrum” function may be provided to the             subsequent optimization algorithm.     -   g) Having an optimization algorithm determine the composition         vector _(j+1)c of a mixture of said nano- or micro-particles         based on portions _(j+1)c_(i) taken from each reservoir R_(i)         during a next iterative step j+1.         -   The optimization algorithm attempts to find a point in the             k-dimensional parameter space, which corresponds to a             composition of said mixture of said nano- or             micro-particles, which exhibits spectral properties that             increasingly better match the desired target spectrum. In             some embodiments the optimization strategy may be based on a             gradient method, a Monte Carlo method, a genetic             optimization algorithms, or other suitable methods, or any             combination thereof.     -   h) Going back to step b) and repeating all subsequent steps         until the RMSE value (or a similar quantity, which is a measure         for the degree with which the spectrum obtained from the         material produced during the last iteration does match the         target spectrum) has been reduced below a predetermined         threshold.     -   i) In general, there may be multiple 0-dimensional points or         lower dimensional sub-spaces (“islands”) within the         k-dimensional parameter space, which correspond to different         composition vectors c but produce spectra which are similarly         well fitting to a given target spectra. Depending on the used         optimization algorithm(s), in particular regarding any contained         true or pseudo random information sources, repeatedly conducting         the described optimization loop may result in similar, or even         identical final vectors c or may produce a plurality of         different vectors c, which still satisfy the chosen condition of         optimality, i.e. the RMSE value or a similar quantity has been         reduced below a predetermined threshold.

It shall be mentioned that from a theoretical standpoint the need for using an optimization algorithm arises because determining a suitable composition vector c to produce a material with a sufficiently close match to the desired spectrum is essentially a nonlinear inverse problem, which can in general not be solved directly.

In some embodiments the optimization process may be aided by computational modeling, which attempts to predict the spectral performance of the material produced during the subsequent iteration. In some embodiments it may be sufficient to only rely on such computation models, or at least during some of the optimization steps.

Once the composition vector c for the specific set of reservoirs R (or more specifically the set of particle properties) and the specific target spectrum is determined, the material can be produced in larger quantities.

There are Several Typical Embodiments, which Correspond to Typical Target Spectra:

In some embodiments the target is to produce a material with a broadband highly reflective spectrum (e.g., reflectivity close to 1), i.e., a broadband bright white, at least within the VIS and NIR range, or even beyond that. Composite materials with such properties and structural elements made thereof are ideal for unconditionally reducing the amount of absorbed solar radiation, thus reducing the amount converted to thermal energy.

In some such embodiments the target spectral property is extended even beyond the NIR range into the FIR range, which in some such embodiments is targeted to be as black as possible, i.e. the FIR reflectance is as close as possible to zero. Such a composite material will not only minimize the absorption of solar radiation and its conversion to thermal energy, but also exhibit improved performance in terms of radiating gained heat, i.e., cooling off once exposure to direct radiation has stopped.

In some embodiments the target is to produce a composite material with a broadband highly absorbing spectrum, i.e. a broadband black material, at least within the VIS and NIR range. Such a composite material will convert most of the incident radiation into heat. Application may include the inside walls of solar-thermal collectors. In some such applications, in addition to being predominantly VIS and NIR black, the material will also be predominantly black in the FIR, which will cause to exhibit increased radiative thermal losses. In some other such applications, in addition to being predominantly VIS and NIR black, the composite material will also be predominantly white in the FIR, which will cause to exhibit reduced radiative thermal losses, i.e. it will stay warm longer after the exposure to radiation stops or is reduced below a level sufficient to retain its temperature.

In some embodiments the target is to produce a composite material of a specific visual color (i.e. not white), but nevertheless with very high broadband NIR reflectivity, as close as possible to unity. The disclosed invention enables to fabricate such “super-cool” yet visually colored composite materials with far better NIR performance compared to existing solutions. Such materials are suitable to give buildings a desired optical appearance while strongly reducing the amount of absorbed solar radiation, thus reducing the amount of retained thermal energy compared to existing solutions.

In some such embodiments the target is to produce a material of a specific visual color (i.e. not white), with relatively high broadband NIR reflectivity, as well as with relatively low reflectivity in the FIR range. Such material can be made to match a desired visual color, still exhibit relatively decreased heat gain under exposure to solar radiation, and will also comparatively rapidly cool off once exposure to solar radiation seizes.

In some embodiments of the disclosed invention the targeted spectral properties of the material are optimized for exposure to electromagnetic radiation of comparable wavelength but other than solar radiation. Such applications may relate to walls of reaction chambers, turbines, ovens, furnaces, or reactors. In many such applications it is desirable to reduce excessive heating of surfaces and materials.

There are Several Additional Embodiments in Terms of the Types of Used Nano- and Micro Particles Although this is not Meant to Imply any Limitations Thereon:

In some embodiments, for example, if the target is to produce predominantly broadband highly reflective materials, some of said nano- and micro-particles may consist of barium sulfate BaSO₄. In some such embodiments the BaSO₄ particles may be derived from finely ground and size-filtered Baryte. In other such preferred embodiments some of said nano- and micro-particles may consist of Kaolinite—Al₂Si₂O₅(OH)₄, and/or zinc sulfide—ZnS, and/or Diopside—MgCa[Si₂O₆], and/or Goethite—α-Fe³⁺O(OH), and/or Rutile—TiO₂ with body-centered tetragonal unit cell, and/or Chrysotile—Mg₃(Si₂O₅)(OH)₄, and/or tin dioxide SnO₂, and/or Calcite—trigonal CaCO₃, and/or hydrated magnesium silicate-monoclinic or triclinic Mg₃Si₄O₁₀(OH)₂, and/or calcium sulfate dihydrate—CaSO₄.2H₂O, and/or titanium dioxide TiO₂, and/or aluminum oxide Al₂O₃, and/or silicon dioxide SiO₂, and/or some sheet silicate minerals including but not limited to Muscovite—KAl₂(AlSi₃O₁₀)(F,OH)₂, and/or other particles derived from similar minerals or synthesized in to form particles with comparable properties.

There are Several Additional Embodiments in Terms of the Used Embedding Material Although this is not Meant to Imply any Limitations Thereon:

In some embodiments discrete populations of nano- and micro particle mixtures obtained as disclosed above are embedded in carrier material which contains or predominantly contains polytetrafluoroethylene (PTFE) or other fluoropolymer.

In some embodiments discrete populations of nano- and micro particle mixtures obtained as disclosed are embedded in carrier material, which contains or predominantly contains polyamide-imide (incl. based on trimellitic anhydride (TMA) and methylene dianiline (MDA)), polyethersulfone (PES), polyether ether ketone (PEEK), polymethyl-methacrylate (PMMA) or other acrylics, or polyurethanes.

In some embodiments discrete populations of nano- and micro particle mixtures obtained as disclosed are embedded in carrier material, which contains or predominantly contains minerals, and/or clay, and/or cement, cementitious material, or consists predominantly of concrete.

There are Several Additional Embodiments in Terms of the Fabricated Structures, which May be Produced According to the Disclosed Invention:

In some embodiments the disclosed material is part of fabricated structural elements. In some such embodiments concrete, cement, cementitious material, clay, or similar minerals is cast into forms, which have said mixture of particles applied to the wall prior to the casting process, such that said particle mixtures become predominantly embedded to the surface regions of the finished concrete, clay, or ceramic elements. This enables to create surface properties which are resistant to damage, scratching, and aging.

In some embodiments concrete, cement, clay, or other ceramics are cast into forms and said mixtures of particles are applied to the still predominantly wet and exposed surfaces. In some of these embodiments said concrete or clay elements are used to erect houses or other buildings and/or are parts to prefabricated houses. Such houses will have enhanced thermal properties, e.g. offer improved living conditions (in terms of inside temperatures) without or with reduced air-conditioning.

In some embodiments the produced composite materials are used to produce predominantly sheets, plates, tiles, etc., which are mounted on the surface of house, including roofs, and are effectively acting as solar radiation shields.

In some embodiments the disclosed composite material is fabricated as a liquid or gel-like layer with sufficiently strong adhesive properties and which may be applied on already erected buildings or on raw building materials.

The application of the foregoing applies to ceramic, bitumen, asphalt, tarmac, clay, mortar, cement, cementitious material, or concrete tiles, blocks, panels, roof elements, columns, girders, arcs, bricks, sheets, layers, or other elements suitable to erect structures or to be mounted onto or applied to the surface of such structures, including but not limited to for housing, storage, protection, bridges, pavements, runways, or other means to support traffic, or other purposes. In some embodiments discrete populations of nano- and micro particle mixtures obtained as disclosed are embedded in carrier predominantly made of asphalt.

In some embodiments, for example, in case of roofs or roof elements, the produced composite material has arbitrary absorbance in the visible spectrum (400 nm to 700 nm) to choose the visible color, high reflectance in the NIR range to reduce the absorption of solar energy, and high absorbance and in the mid IR and FIR, which enables improved thermal emittance. This FIR property support the relatively rapid loss of heat, e.g. during cloudless the nights.

In some embodiments, for example, in case of walls, the produced composite material has arbitrary absorbance in the visible spectrum (400 nm to 700 nm) to choose the visible color, high reflectance in the NIR range to reduce the absorption of solar energy, as well as high reflectance in the MIR and FIR. Walls are typically exposed to mid and far IR (thermal) radiation predominantly diffusely emitted by surround soil, vegetation, or other building and thus this targeted spectral property reduces heat gain also in this spectral range.

The disclosed method enables in some such embodiments, such as those related to buildings, composite materials, which exhibit what may be referred to as “super-cool” properties due to the much-improved approximation of ideal NIR and FIR properties compared to existing approaches and materials.

In some embodiments the disclosed composite material is applied onto predominantly metallic elements. In some embodiments composite materials with discrete populations of nano- and micro particle mixtures obtained as disclosed above, are applied to the surface of vehicles, including automobiles, busses, or trucks, in order to reduce heat gain while exposed to solar radiation.

In some embodiments composite materials with discrete populations of nano- and micro particle mixtures obtained as disclosed above are applied to the surface of satellites or space craft to reduce heat gain while exposed to solar radiation.

In some embodiment, such as defense-related applications, a goal is not to enhance or reduce heat gain but rather to achieve an improved spectral match of vehicles, buildings, or other structures with the surrounding environment. For example, many military and defense related structures, such as concrete bunkers, garages, or silos etc. are frequently painted with various camouflage patterns and in colors matching the direct environment, in order to make visual detection, both by humans or by suitable detection systems operating in the VIS range, more complicated. However, such concrete structures will still have distinct NIR reflectance spectra (NIR “color”) based on their specific composition, which is in general considerably different from surrounding soil or rocks. Thus, detection by aircraft, drone, or satellite based broadband or NIR (“multispectral”) long-range imaging systems is relatively easy, since the erected structures provide in general a clear contrast in said wavelength range.

The disclosed method allows to produce composite materials, from which such structures can be made, or which can be applied to or mounted onto the surface of such structures, which spectrally more closely resemble their environment also in the NIR and potentially FIR range. This can delay and complicate said reconnaissance and detection of said structures from aircraft or satellites. In addition, such properties can also result in a better matched thermal behavior.

In some such embodiments a custom-made layer of materials produced according to the disclosed method is produced in the field. In such cases the VIS and NIR spectral properties can be measured directly on the site or in the close vicinity where a bunker or similar structure is built, a material containing said tailored mixtures of nano- and/or micro particles is produced, for example on a vehicle equipped with the required instruments and equipment for this purpose, and applied to the structure. In some embodiments the material may be applied as a relatively thick spray coating. In other embodiments sheets may locally be produced, which are then mounted onto the surface of said structure.

In yet some other related embodiments spectral measurements are taken at various typical sites, where it is expected that structures with such enhanced detection avoidance capabilities may be needed, and then elements for buildings are pre-produced using sets of composite materials, which are spectrally relatively closely matched as disclosed to said sites. In some such embodiments these may be elements predominantly made from concrete. In some other such embodiments these prefabricated sets of composite materials may be claylike materials, which can be applied to concrete structures, which have previously been erected.

In some embodiments composite materials with tailored discrete populations of nano- and micro particle mixtures obtained as disclosed above, are applied to the surface of vehicles, including tanks, troop carriers, trucks, or other combat vehicles in order to achieve an improved spectral matching both in VIS and NIR relative to an environment wherein said vehicles operate, thus complicating and/or delaying ground-, air-, or space-based detection (including homing systems missiles). In some such embodiments the surface of said vehicles is effectively permanently prepared to have certain broadband spectral properties. In some other such embodiments the surface of said vehicles is predominantly temporarily prepared for a relatively short amount of time, typically on the order of hours or days, but well matched to the broadband spectral properties of the environment, which is expected to be subsequently encountered, e.g. based on the terrain, which is to be traversed. In some such embodiments previous ground-, air-, or space-based reconnaissance missions have at least in part obtained the desired target spectra by measuring typical NIR spectra of the terrain, which is to be traversed.

Although the invention has been disclosed in the context of certain embodiments and examples, it will be understood by those skilled in the art that the invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses and obvious modifications and equivalents thereof. Accordingly, the invention is not intended to be limited by the specific disclosures or preferred embodiments herein. 

What is claimed is:
 1. A method of spectrally camouflaging a structure with a surrounding environment at least within the VIS and NIR range comprising: measuring a broadband spectral reflectance at least within the visible (VIS) and Near Infrared (NIR) range at one or more locations within the vicinity of a structure; providing a plurality of distinct populations of nano- or micro-particles and/or of nano- or micro-cavities, a first population of said plurality of distinct populations comprising one or more of distinct size distributions, shape distributions, chemical compositions, crystal structures, and crystallinity distributions, relative to a second population of said plurality of distinct populations; forming a composite material by embedding a mixture of the first population and the second population in a carrier material, wherein forming comprises selecting a composition ratio of the first population to the second population, and selecting a density of the mixture relative to the carrier material, such that the resulting broadband spectral reflectance of said composite material at least within the VIS and NIR range approximates said measured broadband spectral reflectance as a result of interference effects between at least some of the nano- or micro-particles and/or nano- or micro-cavities of the plurality of distinct populations; and using the composite material on at least part of a surface of the structure.
 2. The method according to claim 1, wherein using the composite material comprises one or more of: coating the at least part of the surface of the structure with the composite material; manufacturing the at least part of the surface of the structure with the composite material; and mounting one or more sheets formed from the composite material onto the structure.
 3. The method according to claim 2, wherein using the composite material comprises coating the at least part of the surface of the structure with the composite material.
 4. The method according to claim 2, wherein measuring comprises at least one of: measuring directly on the site or in the close vicinity of said structure; and measuring in the vicinity of said structure using air-, or space-based multi-spectral long-range imaging systems; wherein using comprises reducing the detectability of said structure via ground-, air-, or space-based multi-spectral long-range imaging systems.
 5. The method according to claim 4, further comprising the structure, wherein the structure comprises a defense related immobile structure.
 6. The method according to claim 4, further comprising the structure, wherein the structure comprises a defense related vehicle.
 7. The method according to claim 6, wherein measuring comprises the measuring in the vicinity of said structure using air-, or space-based multi-spectral long-range imaging systems.
 8. The method according to claim 7, wherein measuring comprises measuring in the expected future environment of said vehicle using air-, or space-based multi-spectral long-range imaging systems, and using comprises reducing the detectability of said vehicle via ground-, air-, or space-based multi-spectral long-range imaging systems when traversing said expected future environment.
 9. The method according to claim 6, wherein measuring comprises measuring typical NIR spectra of terrain which is to be traversed by the vehicle.
 10. The method according to claim 6, wherein using comprises temporarily using the composite material on the vehicle.
 11. The method according to claim 2, wherein forming comprises producing the composite material on site or in close vicinity to the structure.
 12. The method according to claim 11, wherein producing comprises producing the composite material on a vehicle.
 13. The method according to claim 2, wherein the composite material comprises a claylike material.
 14. The method according to claim 2, wherein measuring comprises measuring more than one objects at more than one site.
 15. The method according to claim 1, wherein said plurality of distinct populations have a spatially constant density distribution throughout the carrier material.
 16. The method according to claim 1, wherein said plurality of distinct populations have a higher density distribution in regions close to the surface of the composite material compared to the inner regions of the composite material.
 17. The method according to claim 1, wherein the carrier material comprises a liquid or gel-like material.
 18. The method according to claim 1, wherein the carrier material comprises at least one of minerals, clay, cement, cementitious material, and concrete.
 19. The method according to claim 1, wherein the carrier material comprises polymers.
 20. A method of spectrally matching a structure with a surrounding environment at least within the VIS and NIR range comprising: measuring a broadband spectral reflectance at least within the visible (VIS) and Near Infrared (NIR) range of an environment of a structure; providing a composite material, the composite material comprising a carrier material, a first population of particles, and a second population of particles, wherein the first population of particles is distinct from the second population of particles, and the composition ratio of the first population to the second population is selected such that a broadband spectral reflectance of the composite material is within the measured broadband spectral reflectance range of the environment of the structure.
 21. The method of claim 20, further comprising using the composite material on at least part of a surface of the structure.
 22. The method according to claim 21, wherein using the composite material comprises one or more of: coating the at least part of the surface of the structure with the composite material; manufacturing the at least part of the surface of the structure with the composite material; and mounting one or more sheets formed from the composite material onto the structure.
 23. The method according to claim 22, wherein using the composite material comprises coating the at least part of the surface of the structure with the composite material.
 24. The method according to claim 22, wherein measuring comprises at least one of: measuring directly on the site or in the close vicinity of said structure; and measuring in the vicinity of said structure using air-, or space-based multi-spectral long-range imaging systems; wherein using comprises reducing the detectability of said structure via ground-,air-, or space-based multi-spectral long-range imaging systems.
 25. The method according to claim 24, further comprising the structure, wherein the structure comprises a defense related immobile structure.
 26. The method according to claim 24, further comprising the structure, wherein the structure comprises a defense related vehicle.
 27. The method according to claim 26, wherein measuring comprises the measuring in the vicinity of said structure using air-, or space-based multi-spectral long-range imaging systems.
 28. The method according to claim 27, wherein measuring comprises measuring in the expected future environment of said vehicle using air-, or space-based multi-spectral long-range imaging systems, and using comprises reducing the detectability of said vehicle via ground-, air-, or space-based multi-spectral long-range imaging systems when traversing said expected future environment.
 29. The method according to claim 26, wherein measuring comprises measuring typical NIR spectra of terrain which is to be traversed by the vehicle.
 30. The method according to claim 26, wherein using comprises temporarily using the composite material on the vehicle.
 31. The method according to claim 22, wherein forming comprises producing the composite material on site or in close vicinity to the structure.
 32. The method according to claim 31, wherein producing comprises producing the composite material on a vehicle.
 33. The method according to claim 22, wherein the composite material comprises a claylike material.
 34. The method according to claim 22, wherein measuring comprises measuring more than one objects at more than one site.
 35. The method according to claim 21, wherein said plurality of distinct populations have a spatially constant density distribution throughout the carrier material.
 36. The method according to claim 21, wherein said plurality of distinct populations have a higher density distribution in regions close to the surface of the composite material compared to the inner regions of the composite material.
 37. The method according to claim 21, wherein the carrier material comprises a liquid or gel-like material.
 38. The method according to claim 21, wherein the carrier material comprises at least one of minerals, clay, cement, cementitious material, and concrete.
 39. The method according to claim 21, wherein the carrier material comprises polymers. 